Move some constants to common (#11004)

### What problem does this PR solve?

As title.

### Type of change

- [x] Refactoring

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Signed-off-by: Jin Hai <haijin.chn@gmail.com>

rag/app/presentation.py

@@ -20,7 +20,7 @@ from io import BytesIO
 
 from PIL import Image
 
-from api.db import LLMType
+from common.constants import LLMType
 from api.db.services.llm_service import LLMBundle
 from deepdoc.parser.pdf_parser import VisionParser
 from rag.nlp import tokenize, is_english